There are many situations while completing or performing remedial work on a well where it becomes necessary to isolate particular zones of a well. One reason for isolating a zone is for performing multiple stage downhole stimulations. Industry available products that will isolate the well bore to prevent passage of fluid to other zones are called “plugs.”
Essentially a plug isolates some part of the well from another part of the well. There are several types of plugs, including bridge plugs and frac (fracture) plugs. A bridge plug or frac plug is placed within the wellbore to isolate upper and lower sections of a zone. Bridge plugs hold pressure from both directions, while a frac plug holds pressure from above but allows upward flow. Plugs may be temporary or permanent.
A plug is removed by drilling or milling through it with a bit or blade in combination with circulating a drilling fluid through well to bring up the debris. In a drilling/milling operation, fluid is circulated from the surface through the bit or mill to flush the debris and cuttings from the well. The fluid carries the cuttings and debris to the surface where it is piped to a return tank.
At times it is necessary to work on these wells in an under-balanced condition where the pressures on the well must be controlled by using a choke or choke manifold. A choke is basically a restriction in the return line to hold pressure against the returning flow stream. With the pump rate being constant, the choke or choke manifold will control the downhole pressure. The larger the choke size/opening, the lower the back pressure and the lower the downhole pressure. Conversely, the smaller the choke size/opening, the higher the back pressure and the downhole pressure.
Chokes can be fixed or adjustable. Fixed chokes, also called positive chokes, are basically an orifice and come in a variety of sizes. An adjustable choke is variable and can be controlled electrically, hydraulically, pneumatically, or manually.
Because of their small openings, both fixed (positive) choke and variable chokes are susceptible to debris blocking. Inadvertent restrictions in the flow path can cause undesirable conditions in the well bore associated with drilling and/or milling operations. A restricted flow stream will reduce the ability of the circulated fluid to carry the debris and cuttings to the surface. This condition is serious as it may result in the pipe becoming stuck in the wellbore.
Plugs can be constructed of various materials, including composite materials and metals, such as brass, steel, aluminum, and cast iron. Depending on the material of the plug, the cuttings and debris may include small particulates and/or large rubber or fibrous shreds. Factors determining the size and composition of the debris and cuttings include the differential pressure across the plug when it is milled or drilled, the size of the mill or bit, and the techniques used to break up the plug.
The amount of debris and cuttings produced is dependent on the pipe diameter, pressure rating, plug style and plug manufacture. Common casing size can range from 2⅜ to 9⅝ inches. For example, a 4½ inch plug can produce 300 cubic inches of loose debris. The number of plugs used in a single well is dependent on the number of zones. It is not uncommon to have as many as 15 plugs in a single well.
When a choke or choke manifold is used during a milling or drilling operation, the debris can cause the choke to plug causing instability in the milling or drilling operation. There are two common practices for choke installations in a plug milling operation. One is a single fixed choke bean located in or at the return tank. The other is a choke manifold.
If a single choke bean method is used, when debris clogs the choke, the well has to be shut-in and milling operations stopped until the choke can be cleaned and put back into service. If a choke manifold is used and debris clogs one of the chokes, that choke can be bypassed to the other parallel choke. In this process, one person typically is cleaning the clogged primary choke while another person is trying to adjust the secondary choke back to the desirable backpressure. Not only does this process require extra manpower, but there is also the possibility that both chokes get clogged at the same time and the well has to be shut-in until a choke is cleaned.
As debris collects on a choke, holding a consistent backpressure can be difficult. The choke is opened farther to compensate for the debris restriction; but as the choke is opened, the debris can dislodge, reducing the backpressure, or the debris could clog further increasing backpressure.
In a drilling/milling operation, it is beneficial to remove the milling shavings before the flow stream reaches the choke. Filters or strainers can be placed upstream of the choke to prevent the debris getting to the choke. However, in such systems, parallel filtering systems with a bypass valving arrangement may be required.
The present invention provides the ability to drill continuously multi-plug zones under most common conditions without interrupting the drilling/milling operation to clear a clogged choke. In addition, the invention provides a compact, modular, single filtering system that is easily rigged and can be cleaned while in service. These and other advantages of the invention will be apparent from the following description.